This study seeks to demonstrate that low consumption of vitamin A compounds and/or marginal vitamin A status is associated with the development of colonic adenomatous polyps and/or colonic adenocarcinoma through a case-control study. The study will sort out by careful assessment of ingestion of preformed vitamin A and carotenoids the relative contribution of these A vitaminers to any protective effect observed. Vitamin A status will be assessed by plasma levels and a functional test for dark adaptation. The confounding variable of the level of induction of microsomal carcinogen metabolizing enzymes will also be investigated by measuring urinary D-glucaric acid excretion because vegetables containing high levels of carotene may also contain natural inducers of this enzyme system. A clinical trial using levels of beta-carotene which could be achieved by dietary intake alone will be instituted in subjects found to have adenomatous polyps and these patients will be followed for 3 years for development of polyps and health hazards associated with beta-carotene ingestion. The use of colonic adenomatous polyp formation, a precancerous condition, for a clinical trial has the advantage of having a short time for development and redevelopment occurs in a significant proportion of patients. The ultimate aim of this research is to assess the efficacy of the use of beta-carotene as a natural inhibitor of a particular form of cancer and the safety of luxus ingestion of beta-carotene with a view toward the formulation of valid National dietary advice concerning a protective diet against cancer.